The present invention relates to pressure regulation, and more particularly to a diaphragm-type valve which controls a process pressure to match a reference pressure signal.
Dome loaded back pressure regulators (BPR) are used in many applications, including many research reactors and gas analytical applications in laboratory environments. A typical laboratory example would be core analysis, which uses a heated reactor containing an earthen core sample. Hydrocarbon products are condensed from the gaseous outflow of the reactor as part of the analysis. A back pressure regulator is typically used to simulate various overburden pressures in the earth, and is installed downstream of the reactor. Typical pressure ranges for these applications is between 340 kPa and 21 MPa (50 psig and 3000 psig)
If an inert gas stream is introduced into the reactor, one of several designs of small back pressure regulators can be used to control the sample pressure. However, for many applications where a significant gas flow is undesirable or cannot be assured, a BPR with true zero-flow capability is required. In some applications, “zero flow” can be defined as a gas flow rate less than approximately 0.1 standard ml/hour (0.003 oz/hour). As another example, in a typical catalyst research application at 6.9 MPa (1000 psig) (assuming a typical system fluid volume in the range of 1 ml (0.06 in.3) to 100 ml (6 in.3), loss of less than approximately 1% of system pressure over 24 hours (with the system inlet blocked off) would be considered “zero flow.”
While several designs are available that can provide true zero flow performance under limited process conditions, there is a need for a BPR design that can provide both precision and maintenance of pressure at true zero flow under a wide range of pressure set-points and temperatures.
One prior art BPR design is the MITY MITE back pressure regulator, formerly sold by Grove and now a Dresser Industries product. This BPR uses a simple soft diaphragm, typically polytetrafluoroethylene (PTFE), over a single outlet orifice to control back pressure. At high pressures, the PTFE is indented by the relatively large outlet hole and can develop a bubble-tight seal. This indentation of the PTFE, while useful in obtaining zero flow at higher pressures, also has real limitations on its low-pressure end and also contributes to imprecision and deadband. Though the MITY MITE design serves as a type of industry standard, there are several other disadvantages, including sealing failures after thermal cycling.
Another BPR design for zero-flow applications is described in U.S. Pat. No. 6,886,591, which modifies the MITY MITE approach by using more flexible diaphragms inside very specifically supported constraints. The '591 patent also describe a plurality of outlet holes. These are commonly sold with diaphragms including Stainless Steel foil, PTFE, PTFE/Glass, polyimide, VITON, and PEEK. This technology brings high precision across a very wide range of flow rates and pressures without significant deadband. However, the device of the '591 patent requires careful selection of the diaphragm for a specific pressure range in order to achieve zero-flow performance.
Another BPR design is disclosed in U.S. Pat. No. 4,846,215 to Barree. As detailed in the patent Barree uses an O-ring with a conventional O-ring groove around the single orifice to improve the sealing. While this approach offers potential for low flow performance due to the soft elastomeric seal, this is a very mechanically stressful environment for the soft O-ring to survive. Experiments suggest that the design of the O-ring groove is very critical for the proper functioning of the device. If the O-ring is fully supported by an inner wall of 80% to 85% of its height (as a standard static O-ring groove is), there can be problems achieving a true zero flow seal due to interference between the diaphragm and the hard inner wall. If an inner support is shortened to avoid interference between the support and the diaphragm, the result can be either a failure to flow or a failure to fully close. Detailed experiments with variable support height revealed that the O-ring can be pulled up out of the groove and contorted into non-circular patterns which block the outlet opening. This can be understood by imagining a sealing point of contact at the top of the o-ring, with differential pressure of 1000 psig across this seal. The mechanical stresses are exerted on the elastomer driving the material toward the center. Normal O-ring seals contain the elastomer, but this design provides the possibility for the O-ring to be extruded over the support wall and contorted inward.
This effect is exacerbated because the stresses on the O-ring cause it to become vertically elongated (oval) by the higher pressure on the outer wall of the O-ring than the inner. This means that the O-ring is further elevated above the support wall. In summary, it is practically difficult to identify a support wall height which will reliably contain the O-ring at a variety of pressures without becoming blocked by the O-ring being extruded above the support wall. Even when an O-ring and groove geometry is identified which appears to function through a wide range of pressures with a limited range of gas flow rates, the flow performance becomes unreliable when a higher flow rate of gas further stresses the O-ring.
Accordingly, there is a need for a high precision back pressure regulator with true zero flow capability over a wide range of pressure set points, flow rates, and temperatures.